This invention relates to methods and apparatus for characterizing and monitoring a web BN in a sheet forming process such as paper making. More particularly, this invention relates to methods and apparatus for forming an image of a web at an initial point (the wet end) and applying image analysis and pattern recognition techniques to the image to produce measurements of two-dimensional spatial information and the intensity and depth profiles of the web.
Many production processes involve the use of sheet-forming apparatus, including the making of paper, thin plastic films, certain types of float glass, and some metal sheet processes. Whether formed by the flow of a slurry onto a fourdrinier apparatus as in paper-making or extruded from a die head as in the production of plastic films, there is an initial formation of a web. The web may be further processed. In paper-making, for example, the web is dried and calendared and may be coated before being taken onto a take-up reel for transportation or storage. In the production of plastic films, on the other hand, the only further processing needed may be the curing of the plastic resin.
Different types of sensors and different types of systems have been developed to measure properties of paper and of the paper web. Some of these systems operate on-line, that is, during the paper-making process. Others are used for off-line analysis, as for quality control or research.
A large majority of these systems are intended for use at the dry end of the web-forming process. While such systems may be useful for some quality control purposes and characterization, the analysis is made after the web has been processed to at least some extent, i.e., in paper-making the web has been dried and calendared and in plastics the web has at least started to cure. Thus, some characteristics unique to the wet end have been altered or lost by the time the system has assessed the web.
Even in systems providing for observation of the wet end, observation of the dry end is either required or recommended. U.S. Pat. No. 5,563,809, issued Oct. 08, 1996 (Williams et al.) assigned to ABB Industrial Systems, Inc., proposes a stationary scanner near the wet or initial end of a web. The stationary detector scans across the web in the cross direction (CD) while the web moves in the machine direction (MD). The result is a raster scan of the web. The detected points are digitized and processed by a look up table and a reference memory. For control of the web manufacturing machine, however, a second detector located near the dry end of the web is needed.
Other systems that provide for detection of a web near the wet or initial end also use a scanning methodology. Due to the use of stationary detectors and the movement of the web, most such scanners scan in a raster pattern. Such scanning results in analyzing and/or detecting much less than 100% of the web.
There is thus a need in the art for methods and apparatus that can provide highly detailed and complete information by which a web can be characterized and by which other goals, such as improved headbox and die flows, can be accomplished.
It is an object of this invention to provide a method and apparatus for characterizing a web in a sheet forming process.
It is also an object of this invention to provide a method and apparatus for making and processing an image of a portion of a web to provide spatial, homogeneity, and intensity characteristics of the web.
It is likewise an object of this invention to provide a method and apparatus for characterizing a web with respect to the depth profile thereof.
It is further an object of this invention to provide a method and apparatus whereby a portion of a web can be imaged to allow processing of the image to provide data regarding spatial information in both the cross direction and machine direction of the web, the homogeneity and intensity thereof, and the topography thereof.
It is moreover an object of this invention to provide a method and apparatus that can provide the foregoing information in real time.
It is another object of this invention to provide one subsystem capable of characterizing a web with respect to homogeneity and intensity data and a second subsystem capable of providing a depth profile of the web at the same point, and to provide a method for correlating these characteristics to enable four-dimensional characterization of the web.
It is also an object of this invention to provide information characterizing a web from which the dynamics of the web-forming material, and the characteristics of the headbox or die can be studied, monitored, and controlled.
These and other objects are provided by a method of forming an image of a web of sheet-forming material near an initial point, digitizing said image to form a digital representation of said image, processing said digital representation to provide output signals representative of the spatial characteristics of said web in a cross direction and machine direction, the intensity profile of said web, and the depth profile of said web.
These and other objects are also achieved by providing apparatus having at least one stroboscopic source of light energy for stroboscopically illuminating a web of sheet-forming material, recording means for recording an image of said web during said illumination, digitizing means for digitizing said image to provide a digital representation thereof, processing means for processing said digital representation using image processing and pattern recognition algorithms, and an output means for providing signals representative of spatial information in the machine and cross directions of the web, the intensity profile of the web, and the, depth profile of the web.